The invention is in the field of semiconductor lasers and relates to a semiconductor disk laser (SDL, or VECSEL—vertical cavity surface emitting laser).
Compared with edge emitting semiconductor lasers, semiconductor disk lasers are distinguished by an improved beam quality with a diffraction-limited beam and a circular intensity profile in conjunction with high output powers. By means of the size of the pumped area, semiconductor disk lasers can be scaled in terms of their power, the high beam quality being maintained at the same time. As a result, by diverse external resonator configurations, it is possible to satisfy a large number of complex requirements such as, for example, tuneability of the emission wavelength or mode-selective laser operation in conjunction with high output powers.
As known from the prior art, see for example M. Kuznetsov et al. “High-power diode-pumped vertical external cavity surface emitting semiconductor lasers with circular TEM00 beams”, IEEE Photonics Technology Letters, Vol. 9, No. 8, page 1063 (1997), the semiconductor body of a semiconductor disk laser consists of three regions that are clearly separated in terms of their function:                A) a mirror region, which is highly reflective to an operating wavelength of the laser (laser wavelength). This region can optionally have further optical properties (such as e.g. a high reflectivity in a second wavelength range, see WO 02/47223 A1).        B) an active region, in which quantum wells (QW) and barrier or spacer layers adjoining them are situated. Further layers for specific functionalities can optionally be situated in the active region between the quantum wells and barrier layers (see e.g. J. Paajaste et al. “High-power and broadly tunable GaSb-based optically pumped VECSELs emitting near 2 μm”, Journal of Crystal Growth 311, page 1917 (2009)). In this case, a thickness of the active region is defined from the beginning of the first to the end of the last barrier layer directly adjoining a quantum well. The thickness of the active region thus corresponds to the extent of the active region in the direction of an optical axis (parallel to the main emission direction, with preference parallel to the growth direction of the semiconductor layers, i.e. perpendicular to the wafer surface) of the semiconductor disk laser. Within the active region, in the case of the semiconductor disk laser, pump radiation is absorbed and laser radiation is generated. If the energy of the pump photons is above the band gap of the quantum wells and below the band gap of the barrier layers surrounding the quantum wells, then the pump light is absorbed only in the quantum wells. This is then referred to as so-called “in-well” pumped semiconductor disk lasers. If the energy of the pump photons is above the band gap of the barrier layers surrounding the quantum wells, pump light is absorbed both in the barrier layers and in the quantum well layers. Due to the layer thickness ratios of the barrier layers to the quantum wells, the main part of the pump light in this case is absorbed into the barrier layers, for which reason this configuration is referred to as “barrier-pumped” semiconductor disk lasers.        C) a window region, through which the laser radiation emerges from the structure and which simultaneously shields the active region as a surface from the surroundings. The window region is constructed from semiconductor layers that are transparent to the laser wavelength (i.e. the absorption is minimal). If an optical pumping process with pump photons having a pump wavelength takes place through the window region, then the latter must also be transparent to the pump wavelength. The semiconductor layers in the window region typically have a larger band gap than the last barrier layer of the active region, in order to prevent a diffusion of charge barriers to the surface e.g. in the case of barrier-pumped semiconductor disk lasers. Furthermore, the window region can be constructed such that it alters the reflectivity for the laser wavelength and/or the pump wavelength (e.g. AR for the laser wavelength in WO 02/47223 A1). In many embodiments, the window region is closed off by a thin cap layer at the interface with air. This cap layer has the task of preventing possible oxidation of underlying semiconductor material. The thickness of the window region is defined by the adjoining active region and by the end of the semiconductor body, i.e. the surface as interface of the semiconductor body with the surroundings. The thickness of the window region thus corresponds to the extent of the window region in the direction of an optical axis (parallel to the main emission direction) of the semiconductor disk laser.        
A total thickness of active region and window region thus corresponds to the extent of the active region and of the window region in the direction of an optical axis (parallel to the main emission direction) of the semiconductor disk laser.
It is likewise known to adapt the length of the semiconductor structure in such a way that for the laser light a resonance within the semiconductor structure and thus a standing wave field for a laser field intensity form in order to increase the absorption of the pump light. Absorption efficiencies of the pump light in the range of 65-95% are typically striven for. By way of example, WO 02/47223 discloses directing the pump light through the active region a second time in order to increase the absorption efficiency.
What is disadvantageous about the semiconductor disk lasers according to the prior art is that the maximum output power in the case of semiconductor disk lasers more particularly in the MIR wavelength range between 1.9 and 2.8 μm is limited in comparison with other laser concepts. Likewise, the scalability of the output power by the size of the pumped area has been limited heretofore.